1. Technical Field
The present invention relates to an image display system, an image display device, and an image data output device.
2. Related Art
In recent years, as an image processing function of a computer is highly improved, there is proposed a system in which a computer performs an image processing, which has been performed by an image display device in the related art, image processing data after the processing is transmitted to an image display device side in a digital format, and an image is displayed by the mage display device, such as a projector (for example, refer to JP-A-2004-69990).
In such an image display device, raster data captured by a frame memory of a video output unit of an image data output device, such as a computer, connected to the image display device, is encoded in a predetermined transmission format between the devices and is then transmitted and input to the image display device through a transmission line, such as a USB, and then the encoded data is decoded so as to realize image display.
In this case, if the raster data of a captured screen is transmitted under the uncompressed state, a large amount of data proportional to display resolution, color depth, and a frame rate flows through a transmission line, which causes a problem in that the load imposed on the transmission line or a driving circuit becomes significantly large.
In addition, even a main processor at the computer side should capture and transmit data corresponding to the entire screen, and accordingly, the load imposed on the main processor at the computer side becomes also large.
In order to solve the problems described above, there has been proposed a technique of reducing a required band in a transmission line by using a method in which only a part of the entire screen that has been updated as compared with a previous frame in a time manner, that is, only differential data, is transmitted. According to the technique, since only the differential part of a screen update is transmitted through a transmission line, the technique is very effective for transmitting screen data having a small amount of movement and a small update frequency or a narrow update range, which is typically displayed as a graphic screen by a computer.
However, in the case where an image data output device, such as a computer, reproduces a so-called moving picture based on a format specified by MPEG (moving picture experts group), for example, and then the moving picture is displayed on an image display device that is connected to the image data output device through a transmission line, it is general that a region where the moving picture is displayed occupies a relatively large area in the entire screen. In addition, if frame update is not made according to the frame rate of the moving picture, display based on the frame rate of the moving picture is not achieved. In this case, since a viewer can see dropping frames or jerky movement on the displayed moving picture, it is not possible to secure the quality of a display image. As a result, in order to display a moving picture, it is necessary to capture and transmit a large amount of data.
Furthermore, in the case where the moving picture is compressed moving picture data based on MPEG2 (ISO/IEC 13818-2) or the like, decoding of the moving picture, color conversion, a scaling, and a rasterizing process such as display processing need to be performed at the image data output device side, which also causes a problem in that the load imposed on the image data output device and the transmission line becomes even larger.